I'm trying to simulate a non-thread safe counter class by incrementing the count in an executor service task and using countdown latches to wait for all threads to start and then stop before reading the value in the main thread.
The issue is that when I run it the System.out at the end always returns 10 as the correct count value. I was expecting to see some other value when I run this as the 10 threads may see different values.
My code is below. Any idea what is happening here? I'm running it in Java 17 and from Intellij IDEA.
Counter.java
public class Counter {
private int counter = 0;
public void incrementCounter() {
counter += 1;
}
public int getCounter() {
return counter;
}
}
Main.java
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Main {
public static void main(String[] args) throws InterruptedException {
ExecutorService executorService = Executors.newFixedThreadPool(10);
CountDownLatch startSignal = new CountDownLatch(10);
CountDownLatch doneSignal = new CountDownLatch(10);
Counter counter = new Counter();
for (int i=0; i<10; i++) {
executorService.submit(() -> {
try {
startSignal.countDown();
startSignal.await();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
counter.incrementCounter();
doneSignal.countDown();
});
}
doneSignal.await();
System.out.println("Finished: " + counter.getCounter());
executorService.shutdownNow();
}
}
It's worth remembering that just because something isn't synchronised correctly, it could still perform correctly under some circumstances, it just isn't guaranteed to do so in every situation, on every JVM, on every hardware.
In other words, there is no reverse guarantee, optimisers for example are free to decide your code can be replaced at little to no cost with a correctly synchronised implementation.
(Whether that is what's actually happening here isn't obvious to me at first glance.)
I have a loop which create a new Thread on each iteration, like so:
for(int i = 0; i < REPEAT; i++) {
new Thread(new MyTask(i)).start();
Thread.sleep(1);
}
private void finalTask() {
//Some code to be executed once every threads stopped running
}
Where MyTask is a class implementing Runnable. My goal is: I would like to run finalTask once every threads stopped. To achieve this, I have tried incrementing a variable by 1 each time a thread finished running, and once this variable was equal to REPEAT, the final task would run. But this didn't work. I've searched on Google and StackOverlow for answers to my problem, but there are very little informations on this and none of them worked as well. There would always be a thread that was running after the final task. How can I do this then?
You can use a CountDownLatch for this. A CountDownLatch is
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
CountDownLatch countDownLatch = new CountDownLatch(REPEAT);
for (int i = 0; i < REPEAT; i++) {
new Thread(new MyTask(i, countDownLatch)).start();
Thread.sleep(1);
}
finalTask(countDownLatch);
I create a CountDownLatch whose count is initialized to the value of REPEAT. I pass this to each of the threads and to the finalTask method.
Each thread after doing its work should call the countDown method of the countDownLatch.
private static class MyTask implements Runnable {
private int i;
private CountDownLatch countDownLatch;
private MyTask(int i, CountDownLatch countDownLatch) {
this.i = i;
this.countDownLatch = countDownLatch;
}
#Override
public void run() {
//Perform some task
System.out.println("Running " + i);
countDownLatch.countDown();
}
}
The first line of the finalTask method should call the await method of the CountDownLatch. This will cause the thread running the finalTask wait till the count of the CountDownLatch reaches 0 i.e., until all threads (REPEAT number of them) has completed and invoked the countDown of the CountDownLatch.
private static void finalTask(CountDownLatch countDownLatch) {
try {
countDownLatch.await(); //this will wait until the count becomes 0.
} catch (InterruptedException e) {
e.printStackTrace(); //handle it appropriately
}
//Some code to be executed once all threads stopped running
System.out.println("All done");
}
Another simple way is to just join() on all the threads and then call finalTask():
Thread tasks[] = new Thread[REPEAT];
for(int i = 0; i < REPEAT; i++) {
tasks[i] = new Thread(new MyTask(i));
tasks[i].start();
}
for (Thread task : tasks) {
for (;;) {
try {
task.join();
break;
}
catch ( InterruptedException e ) {
// catch code here
}
}
}
finalTask();
Note there's almost more code used to handle the possible InterruptedException from the join() method call than used to implement the rest of the processing.
You can put them into CompletableFutures and then use whenComplete() .
CompletableFuture[] all =
IntStream.range(0, REPEAT+1).
.mapToObj(i -> CompletableFuture.supplyAsync(new MyTask(i)))
.toArray(CompletableFuture[]::new) ;
CompletableFuture.allOf(all).whenComplete((r, t) -> {
// your code here
}) ;
In my program most of the time tasks are rarely submitted to the executor, yet they don't cease completely. There are periodic bursts when many tasks are submitted at once.
Even though allowCoreThreadTimeOut is set and only one thread would be enough most of the time, the redundant executor threads don't stop.
This is because of the fairness of the executor's blocking queue: when multiple threads wait for it, all have equal chance to get a task and their idle time doesn't grow significantly.
Is there a workaround? For example, a queue that in case of multiple waiting threads returns in the thread with lowest id?
public class ShrinkTPE {
public static void main(final String[] args) throws Exception {
final ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors
.newFixedThreadPool(NTHREADS);
executor.setKeepAliveTime(ALIVE_TIME, TimeUnit.SECONDS);
executor.allowCoreThreadTimeOut(true);
// thread alive time is 10s
// load all threads with tasks at start and every 12s
// also submit one task each second
for (int i = 0;; i++) {
int j = 0;
do {
if (false && !mostThreadsUnused(i))
break;
final int i2 = i, j2 = j;
executor.submit(new Callable<Void>() {
#Override
public Void call() throws Exception {
System.out.println(""
+ Thread.currentThread().getName() + " " + i2
+ " " + j2);
Thread.sleep(300);
return null;
}
});
} while (mostThreadsUnused(i) && ++j < NTHREADS);
Thread.sleep(1000);
System.out.println();
}
}
private static boolean mostThreadsUnused(final int i) {
return i % (ALIVE_TIME + 2) == 0;
}
private static final int NTHREADS = 5;
private static final int ALIVE_TIME = 10;
}
final ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newFixedThreadPool(N_THREAD);
You are using fixedThreadPool and that means, that pool will have N_THREAD number of threads constantly all the time. allowCoreThreadTimeout is neglected here.
Use different thread pool, perhaps CachedThreadPool? It will reuse existing threads, but it will spin up additional threads if you submit new task to the pool and there will be no idle thread.
Idle threads dies after X amount of time (default 60 seconds of idle)
The official JDK implementation of newCachedThreadPool is as follows. You can simply call that constructor directly if you want to set a maximum thread pool size or customized the keepAliveTime or use a different queue.
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
I have used a similar program as below to achieve multithreading to run parallel process but before completing few process my thread is moving to another (or) it is not completing the process completely, I am write some 5 files in parallel with data per thread where out of 5 sometimes 4 files are only writing. Please see the same code I refer,
private static final Random PRNG = new Random();
private static class Result {
private final int wait;
public Result(int code) {
this.wait = code;
}
}
public static Result compute(Object obj) throws InterruptedException {
int wait = PRNG.nextInt(3000);
Thread.sleep(wait);
return new Result(wait);
}
public static void main(String[] args) throws InterruptedException, ExecutionException
{
List<Object> objects = new ArrayList<Object>();
for (int i = 0; i < 1000; i++) {
objects.add(new Object());
}
List<Callable<Result>> tasks = new ArrayList<Callable<Result>>();
for (final Object object : objects) {
Callable<Result> c = new Callable<Result>() {
#Override
public Result call() throws Exception {
return compute(object);
}
};
tasks.add(c);
}
ExecutorService exec = Executors.newCachedThreadPool();
// some other exectuors you could try to see the different behaviours
// ExecutorService exec = Executors.newFixedThreadPool(3);
// ExecutorService exec = Executors.newSingleThreadExecutor();
try {
long start = System.currentTimeMillis();
List<Future<Result>> results = exec.invokeAll(tasks);
int sum = 0;
for (Future<Result> fr : results) {
sum += fr.get().wait;
System.out.println(String.format("Task waited %d ms",
fr.get().wait));
}
long elapsed = System.currentTimeMillis() - start;
System.out.println(String.format("Elapsed time: %d ms", elapsed));
System.out.println(String.format("... but compute tasks waited for total of %d ms; speed-up of %.2fx", sum, sum / (elapsed * 1d)));
} finally {
exec.shutdown();
}
}
May I know any better solution we can do for multi-threading to achieve once the process completes the thread should exit out from the process and I am using Java8,
Updated process code,
public String compute(String obj) throws InterruptedException {
MyProcess myProc=new MyProcess(writeFiles(obj));
myProc.generateReport();
}
public void processMethod() {
List<Callable<String>> tasks = new ArrayList<Callable<String>>();
for (final String object : list) {
Callable<String> c = new Callable<String>() {
#Override
public String call() throws Exception {
return compute(object);
}
};
tasks.add(c);
}
ExecutorService exec = Executors.newCachedThreadPool();
try {
long start = System.currentTimeMillis();
List<Future<String>> results = exec.invokeAll(tasks);
String sum=null;
}
finally {
exec.shutdown();
}
try {
exec.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
} catch (InterruptedException e) {
}
}
Consider the writeFiles will read and write data from database to local file which is huge in memory and need to compare the 5 files which contains difference, where in this case for one time all the files are getting written and for others only one file is getting written and total time of thread is getting shared to all the pool-threads and within the time duration it is not possible to write all the files.
This is because a Future whether is executed in concurrently or sequentially relies on the ExecutorService. so if you change the Executors.newCachedThreadPool() to Executors.newSingleThreadExecutor(), then the tasks is executed in sequentially rather than concurrently, then the elapsed time is almost the same with the total of wait time. for example:
List<Callable<Result>> tasks = asList(() -> compute(null), () -> compute(null));
ExecutorService exec = Executors.newSingleThreadExecutor();
try {
long start = System.currentTimeMillis();
List<Future<Result>> results = exec.invokeAll(tasks);
int sum = 0;
for (Future<Result> fr : results) {
sum += fr.get().wait;
System.out.println(String.format("Task waited %d ms",
fr.get().wait));
}
long elapsed = System.currentTimeMillis() - start;
System.out.println(elapsed / sum);
// ^--- 1
} finally {
exec.shutdown();
}
AND you can see it in java.util.concurrent package summary in detailed as further:
Executor is a simple standardized interface for defining custom thread-like subsystems, including thread pools, asynchronous I/O, and lightweight task frameworks. Depending on which concrete Executor class is being used, tasks may execute in a newly created thread, an existing task-execution thread, or the thread calling execute, and may execute sequentially or concurrently.
What is a way to simply wait for all threaded process to finish? For example, let's say I have:
public class DoSomethingInAThread implements Runnable{
public static void main(String[] args) {
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread());
t.start();
}
// wait for all threads' run() methods to complete before continuing
}
public void run() {
// do something here
}
}
How do I alter this so the main() method pauses at the comment until all threads' run() methods exit? Thanks!
You put all threads in an array, start them all, and then have a loop
for(i = 0; i < threads.length; i++)
threads[i].join();
Each join will block until the respective thread has completed. Threads may complete in a different order than you joining them, but that's not a problem: when the loop exits, all threads are completed.
One way would be to make a List of Threads, create and launch each thread, while adding it to the list. Once everything is launched, loop back through the list and call join() on each one. It doesn't matter what order the threads finish executing in, all you need to know is that by the time that second loop finishes executing, every thread will have completed.
A better approach is to use an ExecutorService and its associated methods:
List<Callable> callables = ... // assemble list of Callables here
// Like Runnable but can return a value
ExecutorService execSvc = Executors.newCachedThreadPool();
List<Future<?>> results = execSvc.invokeAll(callables);
// Note: You may not care about the return values, in which case don't
// bother saving them
Using an ExecutorService (and all of the new stuff from Java 5's concurrency utilities) is incredibly flexible, and the above example barely even scratches the surface.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class DoSomethingInAThread implements Runnable
{
public static void main(String[] args) throws ExecutionException, InterruptedException
{
//limit the number of actual threads
int poolSize = 10;
ExecutorService service = Executors.newFixedThreadPool(poolSize);
List<Future<Runnable>> futures = new ArrayList<Future<Runnable>>();
for (int n = 0; n < 1000; n++)
{
Future f = service.submit(new DoSomethingInAThread());
futures.add(f);
}
// wait for all tasks to complete before continuing
for (Future<Runnable> f : futures)
{
f.get();
}
//shut down the executor service so that this thread can exit
service.shutdownNow();
}
public void run()
{
// do something here
}
}
instead of join(), which is an old API, you can use CountDownLatch. I have modified your code as below to fulfil your requirement.
import java.util.concurrent.*;
class DoSomethingInAThread implements Runnable{
CountDownLatch latch;
public DoSomethingInAThread(CountDownLatch latch){
this.latch = latch;
}
public void run() {
try{
System.out.println("Do some thing");
latch.countDown();
}catch(Exception err){
err.printStackTrace();
}
}
}
public class CountDownLatchDemo {
public static void main(String[] args) {
try{
CountDownLatch latch = new CountDownLatch(1000);
for (int n=0; n<1000; n++) {
Thread t = new Thread(new DoSomethingInAThread(latch));
t.start();
}
latch.await();
System.out.println("In Main thread after completion of 1000 threads");
}catch(Exception err){
err.printStackTrace();
}
}
}
Explanation:
CountDownLatch has been initialized with given count 1000 as per your requirement.
Each worker thread DoSomethingInAThread will decrement the CountDownLatch, which has been passed in constructor.
Main thread CountDownLatchDemo await() till the count has become zero. Once the count has become zero, you will get below line in output.
In Main thread after completion of 1000 threads
More info from oracle documentation page
public void await()
throws InterruptedException
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.
Refer to related SE question for other options:
wait until all threads finish their work in java
Avoid the Thread class altogether and instead use the higher abstractions provided in java.util.concurrent
The ExecutorService class provides the method invokeAll that seems to do just what you want.
Consider using java.util.concurrent.CountDownLatch. Examples in javadocs
Depending on your needs, you may also want to check out the classes CountDownLatch and CyclicBarrier in the java.util.concurrent package. They can be useful if you want your threads to wait for each other, or if you want more fine-grained control over the way your threads execute (e.g., waiting in their internal execution for another thread to set some state). You could also use a CountDownLatch to signal all of your threads to start at the same time, instead of starting them one by one as you iterate through your loop. The standard API docs have an example of this, plus using another CountDownLatch to wait for all threads to complete their execution.
As Martin K suggested java.util.concurrent.CountDownLatch seems to be a better solution for this. Just adding an example for the same
public class CountDownLatchDemo
{
public static void main (String[] args)
{
int noOfThreads = 5;
// Declare the count down latch based on the number of threads you need
// to wait on
final CountDownLatch executionCompleted = new CountDownLatch(noOfThreads);
for (int i = 0; i < noOfThreads; i++)
{
new Thread()
{
#Override
public void run ()
{
System.out.println("I am executed by :" + Thread.currentThread().getName());
try
{
// Dummy sleep
Thread.sleep(3000);
// One thread has completed its job
executionCompleted.countDown();
}
catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
}
try
{
// Wait till the count down latch opens.In the given case till five
// times countDown method is invoked
executionCompleted.await();
System.out.println("All over");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
If you make a list of the threads, you can loop through them and .join() against each, and your loop will finish when all the threads have. I haven't tried it though.
http://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#join()
Create the thread object inside the first for loop.
for (int i = 0; i < threads.length; i++) {
threads[i] = new Thread(new Runnable() {
public void run() {
// some code to run in parallel
}
});
threads[i].start();
}
And then so what everyone here is saying.
for(i = 0; i < threads.length; i++)
threads[i].join();
You can do it with the Object "ThreadGroup" and its parameter activeCount:
As an alternative to CountDownLatch you can also use CyclicBarrier e.g.
public class ThreadWaitEx {
static CyclicBarrier barrier = new CyclicBarrier(100, new Runnable(){
public void run(){
System.out.println("clean up job after all tasks are done.");
}
});
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
Thread t = new Thread(new MyCallable(barrier));
t.start();
}
}
}
class MyCallable implements Runnable{
private CyclicBarrier b = null;
public MyCallable(CyclicBarrier b){
this.b = b;
}
#Override
public void run(){
try {
//do something
System.out.println(Thread.currentThread().getName()+" is waiting for barrier after completing his job.");
b.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
To use CyclicBarrier in this case barrier.await() should be the last statement i.e. when your thread is done with its job. CyclicBarrier can be used again with its reset() method. To quote javadocs:
A CyclicBarrier supports an optional Runnable command that is run once per barrier point, after the last thread in the party arrives, but before any threads are released. This barrier action is useful for updating shared-state before any of the parties continue.
The join() was not helpful to me. see this sample in Kotlin:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
}
})
The result:
Thread-5|a=5
Thread-1|a=1
Thread-3|a=3
Thread-2|a=2
Thread-4|a=4
Thread-2|2*1=2
Thread-3|3*1=3
Thread-1|1*1=1
Thread-5|5*1=5
Thread-4|4*1=4
Thread-1|2*2=2
Thread-5|10*2=10
Thread-3|6*2=6
Thread-4|8*2=8
Thread-2|4*2=4
Thread-3|18*3=18
Thread-1|6*3=6
Thread-5|30*3=30
Thread-2|12*3=12
Thread-4|24*3=24
Thread-4|96*4=96
Thread-2|48*4=48
Thread-5|120*4=120
Thread-1|24*4=24
Thread-3|72*4=72
Thread-5|600*5=600
Thread-4|480*5=480
Thread-3|360*5=360
Thread-1|120*5=120
Thread-2|240*5=240
Thread-1|TaskDurationInMillis = 765
Thread-3|TaskDurationInMillis = 765
Thread-4|TaskDurationInMillis = 765
Thread-5|TaskDurationInMillis = 765
Thread-2|TaskDurationInMillis = 765
Now let me use the join() for threads:
val timeInMillis = System.currentTimeMillis()
ThreadUtils.startNewThread(Runnable {
for (i in 1..5) {
val t = Thread(Runnable {
Thread.sleep(50)
var a = i
kotlin.io.println(Thread.currentThread().name + "|" + "a=$a")
Thread.sleep(200)
for (j in 1..5) {
a *= j
Thread.sleep(100)
kotlin.io.println(Thread.currentThread().name + "|" + "$a*$j=$a")
}
kotlin.io.println(Thread.currentThread().name + "|TaskDurationInMillis = " + (System.currentTimeMillis() - timeInMillis))
})
t.start()
t.join()
}
})
And the result:
Thread-1|a=1
Thread-1|1*1=1
Thread-1|2*2=2
Thread-1|6*3=6
Thread-1|24*4=24
Thread-1|120*5=120
Thread-1|TaskDurationInMillis = 815
Thread-2|a=2
Thread-2|2*1=2
Thread-2|4*2=4
Thread-2|12*3=12
Thread-2|48*4=48
Thread-2|240*5=240
Thread-2|TaskDurationInMillis = 1568
Thread-3|a=3
Thread-3|3*1=3
Thread-3|6*2=6
Thread-3|18*3=18
Thread-3|72*4=72
Thread-3|360*5=360
Thread-3|TaskDurationInMillis = 2323
Thread-4|a=4
Thread-4|4*1=4
Thread-4|8*2=8
Thread-4|24*3=24
Thread-4|96*4=96
Thread-4|480*5=480
Thread-4|TaskDurationInMillis = 3078
Thread-5|a=5
Thread-5|5*1=5
Thread-5|10*2=10
Thread-5|30*3=30
Thread-5|120*4=120
Thread-5|600*5=600
Thread-5|TaskDurationInMillis = 3833
As it's clear when we use the join:
The threads are running sequentially.
The first sample takes 765 Milliseconds while the second sample takes 3833 Milliseconds.
Our solution to prevent blocking other threads was creating an ArrayList:
val threads = ArrayList<Thread>()
Now when we want to start a new thread we most add it to the ArrayList:
addThreadToArray(
ThreadUtils.startNewThread(Runnable {
...
})
)
The addThreadToArray function:
#Synchronized
fun addThreadToArray(th: Thread) {
threads.add(th)
}
The startNewThread funstion:
fun startNewThread(runnable: Runnable) : Thread {
val th = Thread(runnable)
th.isDaemon = false
th.priority = Thread.MAX_PRIORITY
th.start()
return th
}
Check the completion of the threads as below everywhere it's needed:
val notAliveThreads = ArrayList<Thread>()
for (t in threads)
if (!t.isAlive)
notAliveThreads.add(t)
threads.removeAll(notAliveThreads)
if (threads.size == 0){
// The size is 0 -> there is no alive threads.
}
The problem with:
for(i = 0; i < threads.length; i++)
threads[i].join();
...is, that threads[i + 1] never can join before threads[i].
Except the "latch"ed ones, all solutions have this lack.
No one here (yet) mentioned ExecutorCompletionService, it allows to join threads/tasks according to their completion order:
public class ExecutorCompletionService<V>
extends Object
implements CompletionService<V>
A CompletionService that uses a supplied Executor to execute tasks. This class arranges that submitted tasks are, upon completion, placed on a queue accessible using take. The class is lightweight enough to be suitable for transient use when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each returning a value of some type Result, and would like to run them concurrently, processing the results of each of them that return a non-null value, in some method use(Result r). You could write this as:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException, ExecutionException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
solvers.forEach(cs::submit);
for (int i = solvers.size(); i > 0; i--) {
Result r = cs.take().get();
if (r != null)
use(r);
}
}
Suppose instead that you would like to use the first non-null result of the set of tasks, ignoring any that encounter exceptions, and cancelling all other tasks when the first one is ready:
void solve(Executor e, Collection<Callable<Result>> solvers) throws InterruptedException {
CompletionService<Result> cs = new ExecutorCompletionService<>(e);
int n = solvers.size();
List<Future<Result>> futures = new ArrayList<>(n);
Result result = null;
try {
solvers.forEach(solver -> futures.add(cs.submit(solver)));
for (int i = n; i > 0; i--) {
try {
Result r = cs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {}
}
} finally {
futures.forEach(future -> future.cancel(true));
}
if (result != null)
use(result);
}
Since: 1.5 (!)
Assuming use(r) (of Example 1) also asynchronous, we had a big advantage. #